Primary FRCA: Pharmacology – IV anaesthetic agents

What are the barbiturates derived from?
Derived from barbituric acid which a condensation product of urea and malonic acid

How is thiopental presented?
* Sodium salt
* Pale yellow powder
* Vial contains sodium carbonate and nitrogen instead of air → give the solution pH 10.5 → improve solubility

What is the dose of thiopental?
3-7 mg/kg IV

What are the uses of thiopental?
Induction of anaesthesia
Status epilepticus

What are the cardiovascular effects of thiopental?
* Dose-dependant reduction in CO, SV and SVR which can → increase in HR

What are the respiratory effects of thiopental?
* Dose-dependant respiratory depression
* May produce laryngospasm and bronchospasm

What are the CNS effects of thiopental?
* Single dose will induce anaesthesia for 5-10 mins
* Reduced CRMO, CBF and ICP
* Antanalgesic at low doses

What are the renal effects of thiopental?
Decrease urine output due to
– Increase ADH release
– Reduced CO

What is the incidence of severe anaphylactic reactions?
1 in 20 000

Which drugs should be avoided in patients with porphyria?
Thiopental and the other barbiturates
Etomidate
Enflurane
Halothane
Lidocaine and prilocaine
Clonidine
Metoclopramide
Hyoscine
Diclofenac
Ranitidine

What is the pKa of thiopental and how much of it is unionized at body pH?
pKa 7.6 so that 60% is unionized

How much of thiopental is protein bound?
80% protein bound

What factors increase the fraction of the free thiopental?
1. (Critically ill) acidotic and reduced plasma protein-binding → increase in unionized form
2. NSAIDs → reduce available protein-binding sites

Describe the elimination of thiopental
Tri-exponential representing distribution to
1. Well-perfused regions (brain and liver)
2. Muscle and skin
3. Hepatic oxidation to inacitve metabolites

IV infusion results in zero-order kinetics due to saturation of hepatic enzymes

What effect does thiopental have on hepatic enzymes?
Induction of cytochrome P450 even after a single dose

How is propofol presented?
Phenolic derivative and is presented as a 1% or 2% lipid-water emulsion (soya bean oil and purified egg phosphatide)

What is the pKa of propofol and how much of it is unionized at body pH?
pKa = 11 and is almost entirely unionized at pH 7.4

What is the dose of propofol?
IV induction: 1-2 mg/kg
Maintenance: 4-8 µg/ml

What are the cardiovascular effects of propofol?
* Decrease in SVR → ↓ BP
* Depressed myocardial contractility
* Bradycardia especially if given with fentanyl or alfentanil
* Reduced sympathetic activity

What are the respiratory effects of propofol?
* Respiratory depression and apnoea

What are the central nervous system effects of propofol?
* Excitatory effects in up to 10% of patients
* Can be used to control status epilepticus
* Anatagonism of D2 receptors → anti-emetic properties

What are the injection site effects of propofol?
Pain on injection into smaller veins
Reduced by lidocaine and injection into big veins

What are the metabolic effects of propofol?
Fat overload syndrome with hyperlipidaemia and fatty infiltration of the heart, liver, kidneys and lungs.

How much of propofol is protein bound?
98% protein bound to albumin

What is the volume of distribution of propofol?
4 L/kg
Largest of all the induction agents

What is the metabolism and elimination of propofol?
Hepatic metabolism (40% conjugated and 60% metabolized to quinol)
Excreted in the urine

What is the half life of propofol and how is it affected by infusion?
Half-life 5-12
Context-senstive half life increases on infusion

At what age can propofol be given as an infusion to children?
Above the age of 16

How is ketamine presented?
Racemic mixture or as the single S(+) enantiomer – 2-3x more potent than the R(-) enantiomer
Acidic solution pH 3.5 – 5.5
Concentrations of: 10, 50 and 100 mg/ml

What is the dose of ketamine?
IV for induction: 1-2 mg/kg
IM for induction: 5-10 mg/kg

IV for analgesia: 0.2 – 0.5 mg/kg

What are the cardiovascular effects of ketamine?
* Sympathetic stimulation → ↑ HR, ↑ BP, ↑CO and ↑ myocardial oxygen consumption

What are the respiratory/airway effects of ketamine?
* RR increased and larygneal reflexes preserved
* Increased muscle tone associated with the jaw may ppt airway obstruction.
* Bronchodilation

What are the central nervous system effects of ketamine?
* Dissociative anaesthesia (EEG dissociation between the thalamocortical and limbic systems)
* Intense analgesia and amnesia
* α waves replaced by θ and δ waves
* Onset of induction is 90 seconds
* Emergence phenomena which can be reduced by benzodiazepines or opioids; less common in young and old.
* CBF, CMRO and ICP are all increased
* Muscle tone is increased +/- jerking

What are the gut effects of ketamine?
Increase in incidence of N&V
Increase in salivation

How is ketamine eliminated?
Plasma concentration falls in bi-exponential fashion due to:
1. Distribution across lipid membranes
2. Due to hepatic metabolism → demethylated to norketamine (30% as potent) → further metabolized to inactive glucuronide metabolites

Excreted in the urine

How much of ketamine is protein bound?
25% protein bound

Which of the IV induction agents is least protein bound?
ketamine

How is etomidate presented?
0.2% solution of pH 4.1 and contains 35% v/v porpylene glycerol

What is the dose of etomidate?
0.3 mg/kg

What are the cardiovascular effects of etomidate?
Slight fall in peripheral vascular resistance but otherwise little cardiovascular disturbance
Histamine release is rare

What are the metabolic effects of etomidate?
* Inhibits 11β hydroxylase and 17αhydroxylase → inhibition of cortical and aldosterone synthesis
* Increase in mortality when used as an infusion in septic patients

What are the injection site effects of etomidate?
pain on injection

What are the central nervous system effects of etomidate?
* Excitatory movements and nausea and vomiting

How much of etomidate is protein bound?
75% bound to albumin

How is etomidate eliminated?
Rapid distribution into tissues
Hepatic metabolism (+/- plasma cholinesterase) and excreted in the urine
May also inhibit plasma cholinesterase

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